1. Field of the Invention
The present disclosure relates to calibration techniques for ensuring color stability of image forming apparatuses.
2. Description of the Related Art
Granularity, surface uniformity, letter quality, color reproducibility (including color stability) and the like are factors involved in the picture quality of image forming apparatuses, but color reproducibility can be said to be particularly important. Humans have memories in regard to the colors they expect based on experience (in particular, for the color of human skin, blue sky, and metals and the like). These colors are called memory colors. When the color of an image that is formed is beyond the allowable range of memory colors, humans experience an undesirable sense of strangeness.
A multidimensional LUT (lookup table) called an ICC (International Color Consortium) profile exists for maintaining the color reproducibility of image forming apparatuses. By employing ICC profiles, it is possible to achieve a match between the colors of an image displayed on a display device and the colors of an image formed on a sheet by an image forming apparatus. These ICC profiles are created based on color measurements of a pattern using a measuring device.
Japanese Patent Laid-Open No. 2004-86013 proposes a measuring device that measures the color of a test pattern formed on a sheet using a spectral type color sensor. The measurement values from the color sensor are converted to spectral reflectivity, and converted to CIE Lab with consideration to tristimulus values or the like. The CIE L*a*b* color space (CIE is the International Commission on Illumination or Commission Internationale d'Eclairage) is known as a color space that is not dependent on any printing machine or printer.
To improve the measurement precision of colors when measuring the colors of a test pattern formed on a sheet while conveying the sheet in an image forming apparatus, a test pattern of a single color is sampled multiple times and the measurement values of the samplings are averaged. In this case, it is necessary to ensure that the single-color test pattern has a size required for sampling multiple times. Furthermore, the measurement positions of the test pattern formed on the sheet change relatively when there is fluctuation in the conveyance speed of the sheet. For this reason, in addition to the number of times of sampling, it is necessary to give consideration to fluctuation in the conveyance speed of the sheet for the size of each of the test patterns. In this way, when the size of the test patterns becomes large, the number of test patterns that can be formed on a single sheet is reduced.